1. Field of the Invention
The present invention relates to a charge control device for controlling the charging of a secondary cell or battery (hereinafter collectively referred to as a “secondary cell” throughout the specification and the claims) by monitoring its charge state, and relates also to a battery pack employing such a charge control device.
2. Description of the Prior Art
FIG. 2 is a diagram showing an outline of the configuration of an example of a conventional battery pack incorporating a charge control device. The battery pack 1′ shown in this figure is composed of a rechargeable secondary cell 2′, a charge control IC 3′ for controlling the charging of the secondary cell 2′ by monitoring its charge state, feed terminals 4a′ and 4b′ to which direct-current electric power is fed from a charger (not shown), a switch device 5′ connected between the feed terminal 4a′ and the positive electrode of the secondary cell 2′, and a sense resistor 6′ connected between the negative electrode of the secondary cell 2′ and the feed terminal 4b′ (ground line). The sense resistor 6′ has a very low resistance (generally a few tens of mΩ), which permits the charge current flowing through the secondary cell 2′ to be detected as a voltage.
The charge control IC 3′ is built by sealing into a single package a first full charge detection circuit 31′ for checking whether the secondary cell 2′ is in the fully charged state or not by comparing its charge voltage with a predetermined reference voltage, a direct-current voltage source 32′ for generating the reference voltage, a second full charge detection circuit 34′ for checking whether the secondary cell 2′ is in the fully charged state or not by comparing the voltages at both ends of the sense resistor 6′, and a control circuit 35′ for controlling the feeding of electric power to the secondary cell 2′ by turning on and off the switch device 5′ according to the output signals of the first and second full charge detection circuits 31′ and 34′.
It is true that, in this battery pack 1′, the charge control IC 3′ can monitor the charge state of the secondary cell 2′ and control its charging in a considerably satisfactory manner.
However, in the charge control IC 3′ configured as described above, the reference voltage generated by the direct-current voltage source 32′ (i.e., the level of the charge voltage at which the first full charge detection circuit 31′ recognizes the fully charged state) is fixed at the time of the fabrication of the circuit. This makes it impossible to cancel factors that cause variations in the reference voltage after the fabrication of the circuit (for example, the influence of stress occurring when the charge control IC 3′ is packaged or mounted on a circuit board) and fabrication-associated variations in the secondary cell 2′ itself, the very target monitored by the charge control IC 3′. Thus, in the charge control IC 3′ configured as described above, to ensure safe charging operation, the level of the charge voltage at which to recognize the fully charged state needs to be set rather low. This makes it impossible to make the most of the charge capacity of the secondary cell 2′.
On the other hand, the second full charge detection circuit 34′ is so configured as to check whether the secondary cell 2′ is in the fully charged state or not by comparing the voltages at both ends of the sense resistor 6′, which is connected externally. Therefore, by setting the resistance of the sense resistor 6′ appropriately, it is possible to vary the level of the charge current at which to recognize the fully charged state. However, since the sense resistor 6′ has a very low resistance, it tends to be influenced by variations in connection and wiring resistances and other factors. This makes it extremely difficult to correct for a variation in the level at which to recognize the fully charged state. Thus, in the charge control IC 3′ configured as described above, to ensure safe charging operation, the level of the charge current at which to recognize the fully charged state also needs to be set rather low. This makes it impossible to make the most of the charge capacity of the secondary cell 2′. Moreover, the sense resistor 6′, connected externally, hampers the scaling-down and cost reduction of the battery pack 1′.